Light absorption measurements of aerosols by use of a photo-thermal interferometer are for example used to determine the concentration of black carbon particles (BC particles) dispersed in air. BC particles are highly efficient light absorbers, which are for example emitted by combustion processes. Light absorption by BC particles is central to climate change, as elevated BC particles are the second man-made contributor (after CO2) to global warming. Moreover, BC particles also affect public health as these particles are inhaled deeply and associated with lung and cardiovascular disease. However, photo-thermal interferometers may also be used for example to measure other absorbing aerosols, e.g. mineral dust and organic compounds, or gases.
In connection with the present invention the term “aerosol” has to be understood as a colloid of fine solid particles or liquid droplets dispersed in air or another gas. Regarding BC particles as mentioned above an aerosol comprises BC particles dispersed in air.
A photo-thermal interferometer exemplified in a Mach-Zehnder configuration which forms the basis of the preamble of claim 1 is for example disclosed in “Photothermal interferometry for aerosol absorption measurements, S. Sjogren, E. Weingartner and H. Burtscher, http://www.nanoparticles.ch/archive/2016_Sjoegren_PO.pdf”. The Mach-Zehnder interferometer is a device with a first light source emitting a light beam which is divided into a reference beam and a probe beam. Both beams pass through an aerosol or gas which is an aerosol or gas sample. The aerosol sample contains air and BC particles dispersed therein. The gas sample may contain an absorbing gas or gases. The probe beam is superimposed by a pump beam which is emitted by a second light source. After passing the aerosol or gas sample the reference beam and the probe beam interfere with each other thereby causing interference patterns which are recorded by a detector.
Photo-thermal interferometry (PTI) is based on the measurement of small temperature changes within the aerosol or gas sample caused by energy emitted by the second light source (pump beam). When the radiation of the pump beam is absorbed by the particles such as the BC particles or by the absorbing gases, the absorbed energy results in a temperature increase of the particles or the absorbing gases, causing heat to be transferred to the surrounding air of the particles in the aerosol sample or within the absorbing gases of the gas sample. The air heats up locally in the area of highest pump beam intensity, and the gas density around the particles in this area is lowered. This results in a local change of the refractive index of the air within the aerosol or gas sample. The refractive index change is measured as a change in the phase shift between the reference beam and the probe beam when reaching the detector.
Based on the measured phase shift of the reference beam and the probe beam an aerosol or gas sample absorption coefficient may be calculated which is linearly related to the measured phase shift and only depends on the power of the second laser source, the pump-probe beam configuration geometry and the thermal properties of the air.
According to theoretical calculations taking only detector noise into account a very low detection limit in the phase change of 0.02 microradians can be achieved with photo-thermal interferometers at 1 Hz bandwidth (Real-time detection of ambient aerosols using photothermal interferometry: Folded Jamin interferometer, Sedlacek, 2006, BNL-77007-2006-JA). However, the actual PTI instrument sensitivity is much lower (Photothermal Interferometric Aerosol Absorption Spectrometry, Sedlacek and Lee, 2007, ISSN: 0278-6826).
It is an object of the present invention to provide a photo-thermal interferometer which enables measurement of the light absorption of aerosols and gases with an increased sensitivity.